D2 is a conserved nucleosomal protein, present throughout the life of Drosophila. It is "histone H2A-like," but is not a simple sequence variant of H2A. It is less abundant than standard H2A, but is still a major chromosomal protein. It is present with a frequency of one molecule per five nucleosomes. The function(s) of D2 is (are) not known. However, since D2 is associated with a nucleosome subset, it could be a determinant of chromatin specialization, perhaps of differential gene expression. We will employ established procedures in cell and molecular biology to extend our investigation of D2. The proposed experiments may provide information about: 1) the distribution of D2 along genetically active regions of polytene chromosomes; 2) the chromosomal location(s) of D2-coding DNA sequences; 3) the possible coupling of D2 synthesis and DNA synthesis. Proteins that resemble D2 have been reported in Tetrahymena, and in various vertebrates. It is thus possible that the questions concerning D2 can be extrapolated to other eukaryotes, perhaps even to man.